1. Field of the Invention
The present invention relates to a light guide used in a facsimile machine, a copying machine, a hand scanner and the like and, more particularly, to a line illuminating device which is incorporated with the light guide therein.
2. Description of the Prior Art
Equipment such as a facsimile machine, a copying machine and a hand scanner is provided with an image reading device such as an image sensor for reading a manuscript. Used as a type of image reading device is a close-contact type image sensor of which the optical path length is short and which can be easily incorporated in the equipment. In this close-contact type image sensor, it is necessary to read the portion of the manuscript to be read by illuminating in excess of the intensity of illumination readable by an illuminating device. In this case, the range to be illuminated is quite long in the main scanning direction (in the longitudinal direction), but is a narrow, belt-like strip in a sub-scanning direction at right angles to the main scanning direction.
An illuminating device using a bar-shaped or plate-shaped light guide for illuminating the above-mentioned long and narrow belt-like range and a manuscript reading device provided with such an illuminating device are disclosed in Japanese Unexamined Patent Publication No. Hei 8-163320 and Hei 10-126581.
This manuscript reading device is, as shown in FIG. 13, provided with a frame 101 which is formed with recesses 102 and 103. The recess 102 is provided therein with a casing 105 which houses a bar-shaped light guide 104 and an opening portion of the recess 102 is closed by a glass plate 106. A basal plate 108 provided with a photoelectric conversion element (sensor) 107 is attached to the frame in the recess 103. The frame 101 also holds a rod lens array 109 therein.
The bar-shaped light guide 104 is made of glass or transparent resin. The cross-sectional shape of the bar-shaped light guide 104 in the direction at right angles to its longitudinal direction is basically rectangular. The bar-shaped light guide 104 has a chamfered C-shaped portion and serves as an emission plane 110. The bar-shaped light guide 104 is also provided with a luminous means (not shown) such as an LED at one end of the longitudinal direction thereof (i.e. the direction at right angles to the paper surface). The light from this luminous means is introduced to the inside of the bar-shaped light guide 104 from one end of the bar-shaped light guide 104. The illuminating light spreading through the bar-shaped light guide 104 is then scattered in a light scattering pattern 111 formed on one side of the bar-shaped light guide. This scattered light is incident on a reading image surface of a manuscript 113 from the emission plane 110 through a cover glass 112 of a manuscript stand. The reflected light is then detected by the photoelectric conversion element 107 through the rod lens array 109 to read the manuscript.
The light scattering pattern 111 is formed by transferring a white coating material to the surface of the light guide by silk screen printing. However, in the case of the silkscreen printing, there is a problem that the size of a transfer dot varies with many factors such as the clogging condition of a screen, temperature, humidity, diluting condition of a solvent, and splash of the coating material due to static electricity. It is therefore not possible to reproduce the optimum light scattering pattern on the surface of many light guides and as a result, the yield rate of production is decreased. Also, in the case of silkscreen printing, there is another problem that modification and production must be repeated until satisfactory uniformity is obtained and thus, time and cost are increased.
To solve such problems as above, a light guide according to the present invention is provided with an uneven surface for scattering at least light on the predetermined surface thereof (a side will also do) except for the light incidence plane and the light emission plane. It is preferable to integrally form an uneven surface when the light guide is injection-molded. The uneven surface may be formed so that the ridgeline thereof is at right angles to the longitudinal direction of the light guide. However, it is preferable to form an uneven surface of which the ridgeline goes in a different direction rather than at right angles to the longitudinal direction of the light guide.
In the light guide according to the present invention, the uneven surface is integrally formed when the light guide is injection-molded. Accordingly, a process for forming a light scattering pattern by silkscreen printing is no longer needed. Thus, it is possible to economically provide the light guide with uniform characteristics. If the ridgeline direction of the uneven surface differs from the direction at right angles to the longitudinal direction of the light guide, it is possible to take out the light reflected at an inclined plane of the uneven surface directly from the light emission plane. Thus, it is possible to increase the intensity of the emitted light.
In the prior art, the scattered light has been prevented from being emitted uselessly by covering the outside of the light guide with a casing. In the present invention, however, it is also possible to secure the necessary intensity of illumination even without the casing if the forming direction of the uneven surface is set to differ from the direction (i.e. a sub-scanning direction) at right angles to the longitudinal direction. Further, by changing an angle of inclination or position of the uneven surface (i.e. an angle between the ridgeline of the uneven surface and the subscanning scanning direction), it is possible to adjust the position at which the intensity of light reaches its peak on a manuscript surface.
According to a line illuminating device of the present invention, the light guide is housed in a casing so that the light emission plane of the light guide is exposed. The light guide is provided at one end thereof with a luminous means and the uneven surface is formed extending from the end provided with the luminous means toward the other end. Since the light guide is covered by the casing, the line illuminating device can be easily handled (for example, it is not necessary to make arrangements for preventing stains and the like on the light guide when assembled). Further, because the casing serves to prevent the scattered light from being emitted uselessly outside the light guide, it is possible to increase the intensity of the emitted light. By changing the length of the light reflecting and scattering uneven surface from the end which is provided with the luminous means toward the other end, it is possible to obtain a uniform light intensity over the full length of the line illuminating device. If the length of the uneven surface is kept fixed and the range of the uneven surface forming area or the angle of inclination of the uneven surface is changed, it is also possible to obtain a uniform light intensity along the full length of the line illuminating device.
The line illuminating device according to the present invention may be constructed without the casing. When the casing is not provided, the cost of the line illuminating device can be lowered.